Diamond and diamond-like carbon (DLC) coatings have great commercial utility for use as protective coatings, especially for electrode coatings, since such coatings can be doped to be electrically conductive, and are highly corrosion resistant even in strong oxidizing solutions in electrochemical applications with high overvoltage (see for example, M. Fryda, Th. Matthee, S. Mulcahy, A. Hampel, L. Schafter, I. Troster, Diamond and Related Materials, 12 (2003) 1950-1956).
Natural diamond is generally comprised of a cubic crystalline form of carbon. DLC, which is a mixture of mostly sp3—hybridized amorphous carbon, has some of the properties of diamond. Historically, DLC has been applied as a protective coating using a chemical vapour deposition (CVD) process, which process is expensive and limited to coating of planar and relatively small, simple surface topographies.
Thus, the discovery by Patricia A. Bianconi, et. al. (J. Am. Chem. Soc., 2004, 126(10), 3191-3202) of the pre-ceramic precursor polymer poly(hydridocarbyne) (PHC) as a source material to form diamond and DLC coatings at atmospheric pressure and relatively low temperature pyrolysis represents a significant new class of material. PHC is a unique polymer which is a structural isomer of polyacetylene, but with a sp3—hybridized, tetrahedrally bound carbon network backbone comprised of [CH]n. Each carbon must contain a hydrogen substituent to prevent conversion to sp2 carbon so as to minimize the ratio of sp2 to sp3 carbon. Bromoform, which has an sp3 tetrahedral structure was used as a starting material.
Thermal decomposition of PHC results in lonsdaleite, a hexagonal form of diamond. Poly(hydridocarbyne) (PHC) is an air-stable solid at room temperature that forms a nanoparticle or colloidal-dispersion in many polar organic solvents. This feature allows for simple, low cost “dip-coatings” of PHC-organic solvent solutions over large, complex surfaces, where said solution can be dried and heated to form adhesive diamond and DLC sub-micron to micron-thick corrosion resistant thin films or conformal coatings.
Thus, there is great interest and utility in synthesizing PHC cost-effectively on a commercial scale.
Additionally, PHC can be used as a precursor material for synthesizing other materials such as, for example, graphite-like nanospheres (see S. Xu, X. B. Yan, X. L. Wang, S. R. Yang and Q. J. Xue, J. Mater. Sci. (2010), 45:2619-2624). The decomposition of PHC can also be used to increase the tensile strength in an exfoliated graphite matrix (see D. V. Savchenko, S. G. Ionov and A. I. Sizov, Inorganic Mat. (2010) vol. 46 (2):132-138.
As carbon, diamond and DLC coatings are highly biocompatible, the application of DLC coatings over implants such as stents, eye and brain electrodes, cochlear devices, pacemakers, defibrillators, and hip, knee, etc. prosthetics is advantageous.
Since such conformal coatings can also be converted to diamond and DLC by light-activation (i.e. UV laser) processes, new applications such as protective DLC coatings over teeth for dental treatment is possible.
DLC coatings can also be made electrically conductive by doping with, for example, nitrogen (i.e. see A. Zeng, E. Liu, S. N. Tan., S. Zhang, J. Gao, Electroanalysis 2002, 14, No. 15-16, pp. 1110-1115), boron (i.e. see M. Fryda, Th. Matthee, S. Mulcahy, A. Hampel, L. Schafer, I. Troster, Diamond and Related Materials 12 (2003) 1950-1956) or aluminum (i.e. see N. W. Khun, E. Liu, J. Nanoscience and Nanotechnology 2010, 10(7), pp 4767-4772).
Alternatively, PHC can be used as a convenient source material for conventional CVD deposition of diamond and DLC coatings by simple heating without addition of hydrogen or an activation procedure. The traditional method of deposition of tetrahedral amorphous carbon contains a portion of sp2 carbon, which tends to contaminate the final CVD deposited diamond or DLC film. However, by controlling the formation of sp2 carbon, such carbon bonding provides for in-situ doping, thereby providing for electrical conductivity.
U.S. Pat. No. 5,516,884 to Bianconi teaches the formation of 3-D tetrahedrally hybridized carbon-based random network polymers where elements such as silicon, germanium, tin, lead and lanthanides can be incorporated into the network backbone. Each carbon atom has one substituent and is linked via three carbon-carbon single bonds into a 3-D network of continuous fused rings. Thermal decomposition of such polymers forms diamond and DLC carbon. Specifically, Bianconi describes the method for making a variety of polycarbynes, including the synthesis of poly(phenylcarbyne-co-hydridocarbyne) in a 99:1 ratio. However, part of the synthesis process involves the use of NaK alloy, an extremely pyrophoric material, under ultrasonic irradiation, in an inert atmosphere, plus various organic solvents, and additional processing and filtration steps, including refluxing with methyl lithium, rendering this low-yield approach highly problematical for commercial production.
Huang et al. (S. M. Huang, Z. Sun, C. W. An, Y. F. Lu and M. H. Hong, 2001, J. App. Phy., 90(5), 2601-2605) use the reductive condensation of a 1,1,1-tricholorotoluene monomer with an emulsion of NaK alloy in tetrahydrofuran under inert atmosphere to synthesize poly(phenylcarbyne). They also provide data on using a pulsed UV laser for converting the poly(phenylcarbyne) to a diamond-like structure.
U.S. Pat. No. 6,989,428 to Bianconi, et al. provides a detailed summary of the prior art for polycarbyne ceramic polymers used to form diamond and diamond-like carbon. Specifically, it discloses details for the synthesis of poly(methyl- and ethyl-silyne) as a precursor of silicon carbide. However, such synthesis again involves a pyrophoric alloy such as NaK, plus a plethora of processing steps involving various organic solvents and long multiple refluxing steps.
Jung-Hwan Hah, et al. in US Patent Application 2006/0115772 A1, in one embodiment, teach the preparation of poly(hydridocarbyne) by the reductive coupling of CHnX34-n, where X3 includes a group VII halogen such as fluorine, chlorine, bromine or iodine, and n is an integer from 1 to 3. Since each halomethane requires one substituent hydrogen to prevent conversion to sp2 carbon, the Hah reference to n=2 and n=3 suggests that these versions would not form poly(hydridocarbyne). They describe dissolving the poly(hydridocarbyne) in an organic solvent to create a polymeric film for forming a hard mask for fine pattern photolithographic applications. Their reductive coupling step still requires a metallic compound such as NaK or methyl lithium, and in some embodiments heat, ultrasonic wave, light or combinations thereof. No further details are disclosed. However, their process appears very similar to that described by Bianconi in U.S. Pat. No. 5,516,884.
Recent developments by Yusuf Nur et al. (Yusuf Nur, Michael W. Pitcher, Semih Seyyidoglu and Levent Toppare, J. Macromolecular Science, Part A, 2008, 45(5), pp 358-363) and US Patent Application 2010/0063248 A1, describe a method for making PHC using the electrochemical polymerization of chloroform. Said approach is simpler, and potentially safer, than that given in U.S. Pat. No. 5,516,884 by Bianconi.
The Nur process involves electrolyzing chloroform in the presence of acetonitrile, with tetrabutylammonium tetrafluoroborate as electrolyte, run at −6V under nitrogen for 4 hours at room temperature. Various additional steps (i.e. refluxing with tetrahydrofuran and LiAlH4 for 12 hours), plus final PHC purification steps using dichloromethane and hexane are required. Said process also produces an undefined “insoluble material”, and generates chlorine gas, with PHC produced at a 30-40% yield.
In a subsequent publication Nur et al. (Yusuf Nur, Halime M. Cengiz, Michael W. Pitcher and Levent K. Toppare, J. Mater. Sci. 2009, 44: 2774-2779) describe the electrosynthesis of PHC from hexachloroethane. The method used was substantially the same as that using chloroform, with the key difference being that the PHC polymer chain length is bigger using hexachloroethane as a starting material. The methods described by Nur et al. using chloroform and hexachloroethane as starting materials, are still complex, and problematical for producing PHC commercially.
Their method operates the electrochemical cell at −6 V, which exceeds the decomposition voltage of most organic solvents (which tend to decompose at less than 3 volts) thus forming unwanted by-products, including toxic gas such as chlorine gas.
The present invention overcomes the prior art limitations for the synthesis of poly(hydridocarbyne).